WASHINGTON • US President Donald Trump's latest tweet about a button on his desk to launch nuclear missiles has raised questions about the safeguards in place, if any, to prevent one person from making an impetuous decision that would lead to catastrophic consequences. Here are the answers to some of the pressing questions on the issue.

Q Can Mr Trump order a nuclear strike at the push of a button?

A There is no button to press, say experts. The process begins with the President opening the "football", or the black satchel that he carries around at all times. The "football" contains the nuclear codes, the contingency plans and the communication device to send those codes.

Only the US President is able to open the satchel, which has an electronic lock. He carries around a separate set of codes on a small card about the size of a credit card, known famously as "the biscuit".

The biscuit has the President's identity code information. Only with those sets of numbers - and only after they are entered in the right order - will the President be able to open the satchel.

Q How long does it take to launch a nuclear-tipped missile?

A Former US secretary of state Hillary Clinton said during her 2016 presidential campaign that it takes "about four minutes" for the US to launch a nuclear weapon after a president's order is issued.

A 2013 report by the United Nations Institute for Disarmament Research said ICBMs are "capable of launching within five minutes" of a president's order. The Minuteman ICBMs, in fact, are named after their ability to launch within minutes when on high alert.

Q Are there any checks and balance on the Commander-in-Chief's powers?

A Mr Mark Fitzpatrick, a nuclear non-proliferation expert at the International Institute for Strategic Studies in Washington, said that ultimately, the sole authority to launch a strike rests with the President.

"There are no checks and balances on the President's authority to launch a nuclear strike," he told BBC News. "But between the time he authorises one and the time it is carried out, there are other people involved."

Q What has Mr Trump said about the US nuclear arsenal?

A In December 2016, President-elect Trump caused alarm when he called for a strengthening of America's nuclear weapons programme.

The US "must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes", he tweeted.

He later had an off-air phone conversation about the tweet with MSNBC TV host Mika Brzezinski, who said Mr Trump told her: "Let it be an arms race. We will outmatch them at every pass and outlast them all."

Q How much is the US spending on its nuclear arsenal?

A The country is in the midst of a US$1 trillion (S$1.3 trillion), 30-year modernisation of its ageing ballistic missile submarines, bombers and land-based missiles, a price tag that most experts say the United States cannot afford.

Russia is also carrying out a modernisation programme, but is not expanding its warhead stockpile. Over 90 per cent of the world's nuclear weapons are in the US and Russia.

Q Which countries in the world have nuclear weapons?

A Today, nine countries hold around 14,900 warheads: the US, China, Russia, Pakistan, India, Britain, France, Israel and North Korea. Five other countries house warheads (Belgium, Germany, Italy, the Netherlands and Turkey), and 23 others are part of nuclear alliances.

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